It is necessary to control the tension of tow material as it is paid out from a creel to a composite placement head. A tensioning system is active if it can provide back tension and has the ability to reverse the tow payout direction. Prior art active tensioning systems use bi-directional electronic servoed tensioners having a servo motor for each lane of tow to supply back tension and the ability to reverse the tow if required to take out slack during machine movements that decrease the distance between the head and the creel. The servoed tensioners add cost and complexity to the tow handling mechanism. A tensioning system is passive if it can only provide back tension and has no ability to reverse the tow payout direction. For pressure vessel and rocket motor case construction, simple passive tensioning systems are widely used for filament winding machine applications. Filament winding is a continuous process of pulling a tensioned fiber band from a plurality of spools in a creel. Filament winding machines wrap a wet resin fiber band or prepreg fiber band onto a smooth body of rotation having only convex surfaces so that no fiber bridging occurs. Such tensioning systems may use a simple spring on a dancer arm, and a braking mechanism on the spool. Filament winding tensioners provide tension levels of several pounds of force or higher since the shapes to be wound are usually symmetric, and are tightly wrapped in a continuous manner without stoppage. With the high tension levels, the effects of spool inertial loads are a manageable component for the passive spring design. Spring based tensioners for filament winding size material spools are not required to operate at a tension level of less than several pounds of force.
In the fiber placement process, it is necessary to place prepreg fiber bands in a discontinuous manner on lay-up tools that have concave areas and near net shape perimeters. The fiber placement process requires frequent starts and stops of the fiber band application. Due to these requirements it is necessary for the fiber placement tensioners to be able to provide low tensions in the magnitude of less than 1 pound, and with a quarter-pound tolerance. This is necessary so the fiber band will not bridge (stretch across the valleys) as it is laminated by a compaction roller onto concave areas of the tool surface. It is not possible at the low tensions used in the fiber placement process to control passive units sufficiently to buffer rapid tow acceleration and deceleration changes and resulting spool inertial loads with fine enough resolution for the fiber placement process. The amount of tension spike buffering from spring loaded dancer rolls on passive tensioners is not adequate for the fiber placement process. Prior art passive tensioning systems cannot adapt to changing spool diameters and changing accelerations and decelerations needed for feeding the tow and stopping the tow during fiber placement operations. When the material spools and tensioners are integrated into the head, it becomes necessary for tensioner and its dancer roll motion to be immune to changing gravity vectors as the head changes its position in the operating zone. Prior art mechanical spring based tensioners are affected by operating orientation. This can result in tension variations from gravity loads affecting the force response of the dancer roll spring.